There is STI (Shallow Trench Isolation) as one of technologies for electrically isolating elements formed on a silicon substrate from one another. The STI is generally formed by forming a trench in Si using as a mask of a Si3N4/SiO2 film, burying SiO2 in the trench, and flattening by CMP (Chemical and Mechanical Polishing).
In the STI, dry etching such as RIE (Reaction Ion Etching) is used for forming the trench, in which electrical leakage is caused through crystal defects (e.g., radiation damage etc.) and shape defects (e.g., RIE damage, heterogeneous film of liner SiO2, void/seam of buried SiO2, etc.) as fining proceeds in the semiconductor device. In order to prevent such problems, there is an attempt for improvement through a pullback by a wet process (a wet etching) using heated phosphoric acid etc. Here, the term “pullback” is a step of removing/retracting a part of an etching mask such as Si3N4 etc., which offers an obstacle at a later step, (in case of STI, a part neighboring the trench, which offers an obstacle in burying SiO2).